The term "near infra-red" is used herein to mean electromagnetic radiation having a wavelength of about 700 to about 1200 nm.
Near infra-red absorbers, that is to say compounds which absorb near infra-red electromagnetic radiation, are known to be useful in a number of applications. For example, such absorbers are useful as infra-red filters used in photography and for protecting sensitive materials from infra-red radiation, and in "inks" used to print characters intended to be read using near infra-red light--for example, in universal product code labels intended to be scanned with an infra-red laser. Such absorbers are sometimes useful as sensitizers to render photographic film sensitive to near infra-red radiation, and some of them can act as charge transfer materials and are thus applied to the photosensitive surfaces used in xerography and in other types of electrophotography.
A wide variety of compounds have been used as infra-red absorbers, including benzthiazoles, metal dithiolenes and phthalocyanines. Benzpyrylium and benzthiopyrylium dyes have also been used for this purpose.
For example, U.S. Pat. No. 3,554,745, issued Jan. 12, 1971, describes the use of 4-aminobenz[b]pyrylium salts as sensitizers for photoconductors.
U.S. Pat. No. 3,617,268, issued Nov. 2, 1971, describes a photosensitive material for electrophotography comprising an organic photoconductive compound and a sensitizer consisting of reaction products of a benzopyrylium salt and a benzopyran.
Canadian Patent No. 1,247,915 discloses a bis(4-benzpyrylium) pentamethine dye used as an infra-red sensitizer. The nuclei each carry a 2-phenyl substituent and a 7-acetyl group.
Reynolds et al., U.S. Defensive Publication No. T-889,023, published Aug. 31, 1971 describes several bis(4-benzpyrylium) pentamethine compounds in which the pentamethine chain is unsubstituted and unbridged. The nuclei have 2-dimethoxyphenyl substituents and may have 7-acetyl substituents.
U.S. Pat. No. 4,714,667 discloses bis(4-pyrylium) and bis(4-benzpyrylium) pentamethine and heptamethine compounds for use in optical information recording media. In these compounds, three of the carbon atoms of the chain form part of a 5- or 6-membered ring, which is not symmetrically disposed in the chain when a pentamethine chain is present.
Japanese Patent Application No. 103,604/82 (Publication No. 220,143/83, published Dec. 21, 1983), discloses a broad class of bis-heterocyclic pentamethine dyes in which the central three carbon atoms of the pentamethine chain form part of a squarylium or croconylium ring. The heterocyclic nuclei can be pyrylium, thiopyrylium, selenopyrylium, benzpyrylium. benzthiopyrylium, benzselenopyrylium, naphthopyrylium, naphthothiopyrylium or naphthoselenopyrylium nuclei, which can be substituted with alkyl, alkoxy, aryl or styryl groups.
Japanese Patent Application No. 60-8730 (Publication No. 167,681/86, published Jul. 29, 1986), discloses bis(4-benz[b]thiopyrylium pentamethine dyes in which the central three carbon atoms of the pentamethine chain for part of a squarylium ring. The dyes are intended for use as infra-red absorbers.
West German Offenlegungsschrift No. 38 34 960 discloses a bis(2-t-butyl-4-benzpyrylium) monomethine dye for use in a photopolymerizable composition.
Infra-red absorbers have the capacity to absorb infra-red radiant energy and convert it to heat, thereby heating any medium in which the infra-red absorber is present, and several applications of such absorbers depend upon such generation of heat. Thus, infra-red absorbers can be used to induce chemical or physical changes in the medium containing them, and such chemical or physical changes can be induced with an appropriate wavelength of light. For example, a layer of infra-red absorber spread over a supporting plate can undergo visually perceptible physical distortions upon illumination by an infra-red laser, so that the layer acts as an optical recording medium, as in a compact disc. For example, U.S. Pat. No. 4,508,811, issued Apr. 2, 1985, describes an optical recording element in which the recording layer comprises a bis(2,6-dialkyl) pyrylium or thiopyrylium squarylium salt.
U.S. Pat. No. 4,555,472, issued Nov. 26, 1985, describes an optical recording member comprising a bis(2,6-diaryl-4-pyrylium) trimethine compound, in which the central carbon atom of the trimethine chain may bear an alkyl or aryl substituent.
Such chemical and physical changes can also be used in thermal imaging; the highly localized heating produced by the infra-red absorber can be used to create a high resolution image. For example, U.S. Pat. No. 4,720,449, issued Jan. 19, 1988, describes a thermal imaging method which comprises heating imagewise a di- or triarylmethane compound possessing within its di- or triarylmethane structure an aryl group substituted in the ortho position to the meso carbon atom with a moiety ring-closed on the meso carbon atom directly through a nitrogen atom, which nitrogen atom is also bound to a group with a masked acyl substituent that undergoes fragmentation upon heating to liberate the acyl group for effecting intramolecular acylation of the nitrogen atom to form a new group in the ortho position, whereby the di- or triarylmethane compound is rendered colored in an imagewise pattern corresponding to the imagewise heating.
U.S. Pat. No. 4,602,263, issued Jul. 22, 1986, and U.S. Pat. No. 4,826,976, issued May 2, 1989, both describe thermal imaging systems for optical recording and particularly for forming color images. This thermal imaging method relies upon the irreversible unimolecular fragmentation of one or more thermally unstable carbamate moieties of an organic compound to effect a visually discernible color shift from colorless to colored, from colored to colorless or from one color to another.
International Patent Application No. PCT/US87/03249 (Publication No. WO 88/04237), published Jun. 16, 1988, describes a different type of thermal imaging system using an element comprising a support formed of a material transparent to radiation of a specific wavelength and having an imaging surface layer heat activatable at an elevated temperature, and a layer of porous or particulate imaging material uniformly coated on the imaging surface layer and exhibiting a cohesive strength which is greater than the adhesive strength between the imaging material and the imaging surface layer. When this element is illuminated with radiation of the specific wavelength, at least one of the materials used in the two layers absorbs this radiation, thus heat activating the imaging surface layer and locking substantially the entire layer of the imaging material to the support when the imaging surface layer cools. After exposure of the element, a peeling force is applied to the imaging material so that in the unexposed areas of the element, the imaging material will peel from the support; however, in the exposed areas, the locking of the imaging material causes this material to be retained upon the support.
In many thermal imaging systems in which a leuco dye is transformed into a colored compound by heat, the leuco dye does not have sufficient absorption at a convenient wavelength to permit it to convert sufficient radiation to heat to effect the color change. For example, in the aforementioned U.S. Pat. Nos. 4,602,263 and 4,826,976, many of the leuco dyes absorb in the ultraviolet. In such thermal imaging systems, it is normally preferred to use a laser as the radiation source, and at present ultraviolet lasers are not well-suited to imaging processes, and such processes are preferably carried out using an infra-red laser. Accordingly, it is preferred to include with the leuco dye an infra-red absorber for converting infra-red radiation into heat, which is transferred to the leuco dye to effect the color change.
Similarly, in the thermal imaging system described in the aforementioned International Patent Application No. PCT/US87/03249, an infra-red absorber may be provided in a layer adjacent the imaging surface layer to assist in converting infra-red radiation into heat.
The requirements for infra-red absorbers for use in thermal imaging systems are stringent. Since the sensitivity and the resolution of the image produced are often affected by the thickness of the layers in the heat-sensitive element (the sensitivity of the system is inversely related to the mass of material required to be heated, and thus inversely related to the thickness of the relevant layers), it is necessary to provide a high degree of absorption of infra-red radiation within a thin layer, sometimes of the order of 1 .mu.m. To produce this degree of absorption in a layer containing the other components required in thermal imaging systems, it is necessary that the infra-red absorber used have a high extinction coefficient, of the order of at least about 100,000, and a low molecular weight. In addition, the absorber should manifest its maximum absorption within the range of about 700-1200 nm. so that it can conveniently be used with existing near infra-red lasers. (In the present state of technology, solid state diode lasers emitting at about 760 to 1200 nm. provide the highest output per unit cost. YAG and other rare earth doped lasers emitting at about 1000-1200 nm. are also useful in thermal imaging processes.)
One major problem with many prior an infra-red dyes is that they have low solubility either in most plastics and/or in semi-polar solvents (for example, methyl ethyl ketone and methylene chloride) from which they need to be deposited to form imaging media such as those used in the aforementioned International Patent Application No. PCT/US87/03249. Thus, it is difficult to dissolve or disperse the absorber in a plastic without forming aggregates and without adversely affecting other properties of the plastic.
Accordingly, despite the numerous prior art attempts to develop near infra-red absorbers, there are still applications of such absorbers, especially in thermal imaging methods, where it is difficult to find a prior art absorber which fulfills all the requirements of high extinction coefficient, low molecular weight, desirable wavelength of maximum absorption and compatibility with other components of the desired medium. In addition, some prior art infra-red absorbers degrade in films too rapidly to be useful in media which need to have a shelf-life of at least several months. Finally, some prior art infra-red absorbers, especially those which include sulfur-containing heterocyclic rings, have undesirable odors, which may be objectionable to users of the dyes when they are employed in a medium which permits escape of the dye into the air or onto the fingers of users.
Accordingly, there is still a need for development of improved near infra-red absorbers.
It has now been found that the solubility of bis(benzchalcopyrylium) squarylium and croconylium dyes in semi-polar solvents and plastics can be substantially improved by providing, on at least one of the benzchalcopyrylium nuclei, a 2-substituent in which a non-aromatic carbon atom is bonded directly to the benzchalcopyrylium nucleus.